1. Technical Field
The present invention relates to pressure regulation, and in particular, to systems and devices that regulate the internal pressure of liquid fuel tanks in high-speed air and space flight vehicles.
2. Description of the Related Art
Flight vehicles that travel at very high altitudes and into the earth's atmosphere require high-rate combustion engines in order to achieve the hypersonic velocities necessary for such flights, generally in range of Mach 4-10. One type of engine for propelling flight vehicles of this type is a supersonic combustion ramjet (“scramjet”) engine. Proper fuel delivery to the engine is critical for any flight vehicle to avoid unintended engine shutdown and failure. However, the high rate at which fuel is combusted in scramjet engines makes fuel delivery a significant challenge both at ignition and during sustained operation. The extreme pressure and temperature environment common in such high-speed applications further complicate fuel delivery.
Valves for metering fuel and other combustible media to engines in aircraft and spacecraft are known in the art, see e.g., U.S. Pat. No. 6,250,602 B1, assigned to the assignee of the present invention. Such valves are capable of accurately and precisely controlling high flow pressurized fuel supply to the engine combustion chamber(s).
Even precision metering valves, however, can lose the ability to accurately meter fuel if pressure loss occurs upstream from the valve, such as if pressure is lost to the primary fuel pump arising from instability in the fuel tank. In liquid fuel systems there is a vacuous space within the fuel tank, known as ullage, where no liquid fuel is present. The ullage varies constantly as fuel is burned during operation of the engine. This change in ullage can cause the internal tank pressure to vary and possibly drop below a minimum pressure level. Should fuel pressurization drop excessively accurate fuel delivery to the engine(s) could be affected and engine performance could be compromised or possibly not sustained.
Pressure regulators have been devised to stabilize the pressure of a fuel tanks, and are generally classified by the actuator mechanism being mechanically or electronically driven. Known mechanical regulators are typically designed for relieving excess pressure and use a simple spring and piston arrangement to open internal space to ambient or other relief lines. Mechanical regulators can be imprecise, particularly at high pressure conditions, due to the effects of large frictional forces and hysteresis. Multiple stages may also be required for large pressure range applications, which add complexity, size and weight to the regulator and further impair accuracy and reliability. Moreover, mechanical regulators generally provide a single pressure set point, which is either fixed or can be varied only by mechanical adjustment to the regulator.
Known electronic regulators, on the other hand, generally can be configured to build up or relieve pressure within a range of pressures in response to electrical input. The set point can be varied easily and the unit size and weight is generally favorably compared to mechanical regulators because of the electronic actuation. Electronic controllers can output pressure in proportion to an electrical input essentially unaffected by changes in supply pressure so that the regulator can provide accurate, high-resolution control of pressure at low flow conditions. However, the inaccuracy of known electronic regulators, particularly at high flow rates, can be unacceptably high for critical applications, such as the fuel systems of high-speed flight vehicles.
Moreover, for any flight vehicle, especially hypersonic vehicles, it is critical to minimize the size and weight of on-board systems. The size and weight of the fuel system can be significantly lessened by using a high energy density fuel tank pressurant system and a low energy density fuel tank made of thinner walls than would be necessary under high pressure. However, this requires the regulator to perform over a very large pressure drop, in the range of 5,000 psia for example, which can be too great for conventional regulators to handle while maintaining accuracy.